Field of the Invention
The present invention relates to turbojet engines and, more particularly, to their accessory gear box, generally known as “Accessory Gear Box” (AGB).
Description of the Related Art
Some of the power generated by a turbojet engine is used to supply energy to various items of its equipment. This power is mechanically obtained from the shaft of the high pressure (HP) spool of the turbojet engine by a power take-off shaft which drives an input shaft in the accessory gear box. This housing contains a certain number of gears connected to items of equipment or accessories, such as, for example, an electric generator, a starter, an alternator, hydraulic pumps for fuel or oil, etc. These various accessories are mechanically driven by the AGB input shaft which, via the gears of the AGB, transfers some of the power obtained from the HP shaft to each of them.
The AGB generally comprises a cylindrical housing, made of an aluminium alloy casting closed at the top and bottom by two substantially parallel walls, thus defining a housing to accommodate the gear-wheels that drive the accessories. Each gear-wheel comprises a shaft able to accommodate the drive shaft of an accessory and one of said gear-wheels is also connected to the AGB input shaft. The various accessories driven by the AGB are mounted directly on the cast housing, the drive shafts of these accessories passing through cutouts made in one of the walls of this housing to drive them and dimensioned so as to allow the corresponding gear wheel to be mounted inside the housing. The gear wheels are positioned in the cast housing so that they are supported on one side by the housing wall opposite the cutout for its drive shaft and, on the other, by a removable cover which attaches to the slotted wall of the housing. As a general rule, although this configuration is not essential, the gear wheel shaft is supported by the outer ring of a roller bearing attached to the uncut bottom wall of the housing and by the outer ring of a ball bearing attached to the cover associated with said accessory.
In a multi-flow turbojet engine the accessory gear box is generally mounted on the fan casing inside the fan compartment in the space delimited by the fan casing on the inside and the nacelle on the outside. It is shaped to fit the curve of the fan compartment, the drive shafts of the accessory machines being oriented along the engine axis. The power take-off shaft on the HP rotor driving the input shaft of the gear box is arranged inside one of the arms of the intermediate casing to which the fan casing is attached.
The hydraulic pumps connected to the accessory gear box constantly rotate while the engine is running and there is no provision for being able to disconnect them. When the pump is inactive, this rotation is only accompanied by minimal circulation of hydraulic fluid, just sufficient to renew it; it therefore creates a significant amount of heat which transfers via conduction to the gear box, which is already at a temperature close to the specified limits.
It would be desirable to prevent the accessory gear box having to absorb this additional power.
It would also be desirable to be able to ventilate the fan compartment while the aircraft is standing still with the engine running.
A proposed solution was to install an insulating material between the accessory and the gear box housing. This insulation serves to reduce the interface temperature for the equipment but does not dissipate the heat. Moreover, the simple insulation solution creates a temperature gradient between the equipment and the gear box if the equipment is colder than said gear box. However, if the equipment itself dissipates the energy that should be evacuated, the insulation only serves to limit the flow of heat via conduction to the accessory gear box and the temperature of the equipment increases until it eventually exceeds the permitted threshold.